1. Field of the Invention
This invention relates to a piece of light transmitting glass having at least one surface area which is matted by surface pits. The invention also relates to a process of producing matted glass comprising forming a surface area of a piece of glass with a population of surface pits.
2. Description of the Related Art
It is well known that light is reflected at the boundary between two media having different refractive indices. Assuming that the media are thick enough that interference effects can be ignored, the amount of normally incident light which will be reflected will be approximately [(n.sub.1 -n.sub.2)/(n.sub.1 +n.sub.2)].sup.2, where n.sub.1 and n.sub.2 are respectively the refractive indices of the two media. In the case of a glass/air interface, we can write n.sub.1 .perspectiveto.1.5 and n.sub.2 .perspectiveto.1, and the expression thus indicates that about 4% of light incident on any glass/air interface will be reflected at that interface, and about 8% will be reflected from the two surfaces of a glass sheet in air.
This reflection is a disadvantage for many purposes. As examples may be cited: instrument dial glasses, for example watch glasses; glass panels covering light emitting diode displays, and more particularly those covering liquid crystal displays; cathode ray tube screens, for example television screens and computer monitor screens; and sheets of glass held in picture frames to protect a photograph, painting or drawing.
It is well known that the amount of light reflected from a glass sheet can be reduced by applying a surface coating of a material having a refractive index intermediate those of air and glass. By selecting the coating material to have a refractive index which is the geometric mean of the refractive indices of the two other media, in this case glass and air, the total amount of light reflected at the air/coating and coating/glass interfaces can be reduced to about 2%, and by selecting the thickness of the coating for interference extinction between the light reflected at the two interfaces, the amount reflected can be reduced even further. However such coatings are difficult and expensive to apply, and materials having the required refractive index are often softer than glass so that they may easily become abraded. Also some such coating materials are inherently coloured, and they can be detrimental to the performance of other coatings which may be required for other purposes.
Since it is the image of the light source on the glass which gives rise to problems in viewing through the glass, it has been proposed not so much to reduce the total amount of light reflected, as to reduce the proportion of reflected light which is reflected specularly. As an example, it is well known to etch the surface of glass so that its surface is matted, in that it is densely populated with pits which act to scatter surface-reflected light. As a result, a high proportion of that light is diffusely rather than specularly reflected. An example of a method of producing such glass is set forth in Glaverbel's British patent specification No. 1 151 931.
Such surface matting also affects light transmitted by the glass, and a proportion of the light transmitted through such a matted area will also be diffuse. This can be an important disadvantage if it is desired to view an object or image located some distance behind the glass surface, since the diffusely transmitted light will make optical resolution of the object or image difficult or impossible. For example it will be appreciated that television screens, especially in the larger sizes which are commercially available, may often be several centimeters thick and any matting treatment performed on the exterior surface of the screen will accordingly impair resolution of the image formed on the interior, coated face of the screen.